I think systems like this use a lot of RAM. You can afford it when you're a stock exchange. You can get high availability from a RAM store if you replicate it o

I think systems like this use a lot of RAM. You can afford it when you're a stock exchange. You can get high availability from a RAM store if you replicate it over multiple machines and react quickly to machine failures. Then you keep a commit log on disk that's append only and avoids seeking.